1. Field of the Invention
The present invention is direct to a hydrogel-based prosthetic device to replace all or part of the nucleus of a spinal disc after it has been surgically removed. More specifically, the present invention is directed to such a device which contains a uniquely advantageous combination of differing structural compositions and geometries.
2. Information Disclosure Statement
The spinal disc is a cartiligeous spinal joint which allows the bending and rotation of the spine. Damage to the spinal disc leads to a dysfunction of the spine, serious pain, and often to a long-term disability of the patient. A typical problem consists of the spinal disc bulging out or herniating so that the nucleus becomes extruded, which then causes a compression of an adjacent nerve and an inflammatory reaction. Presently, it is typical to immobilize the joint by fusing the neighboring vertebrae using various instruments and techniques. It is also common to remove the nucleus or its part in a procedure called a laminectomy.
All present surgical interventions, whether laminectomy or fusion of adjacent vertebrae, lower the functionality of the spine in some way. For that reason it is desirable to try to develop a prosthetic for the spinal disc or its part. This is, however, extremely difficult. The spine is a very complex part of the body and its proper function is dependent on proper coordination of the function of all the parts, including the spinal discs. The system needs to withstand complex stresses, including various angles of bending, pressure, shear, and twisting. The spinal disc must also function as a shock and vibration absorber. And finally, a spinal disc must allow the transport of the nutrients and metabolic products needed for its health and survival. These complex requirements elucidate the complex structure of the spinal disc.
The spinal disc is made of a hydrogel-like core called Nucleus Pulposus (NP), and an outer sheath called Annulus Fibrosus (AF). The AF consists mainly of collagen fibers which are organized into many crisscrossed layers somewhat like layers in automotive tires. This configuration ensures significant resistance against radial stress and inner over-pressure, while allowing significant deformation during twisting and bending. The NP is located within the AF. The NP is a hydrogel-like composite made of proteoglycane and collagen, with a uniform water content of more than 90% of total mass. Its composition is similar to the AF except it has a lower collagen content and a higher content of proteoglycane and water. Aside from that, the NP is isotropic, while the AF is significantly anisotropic. The NP is connected to the AF, and the transition between these two bodies is gradual.
The areas of adjacent vertebrate bodies can also be thought of as parts of the intra-vertebral joint system. These areas are covered with cartilage consisting of a collagen matrix filled with glycoprotein and water. Just like the NP and the AF, these cartilages are a living tissue that contains approximately 2–5% of live cells needed to renew the cartilage. The collagen fibrilles of the AF are attached to the vertebrate area cartilage. The NP is attached to the AF, but not to the vertebrate area cartilage. This arrangement is important for the mobility and proper function of the intra-vertebral joint.
The function of the spinal disc can be compared to the function of a tire in a car. In this case the “tire” itself is made of the AF, while the NP functions like the compressed air in the tire. The NP has viscoelastic mechanometric characteristics. Aside from that it is, like air, capable of changing its volume with a changing load. This is achieved by the change of the water content due to outside pressure. Resistance produced by the nucleus to the reduction of water content by mechanical pressure (“wringing”) is called “swelling pressure”. Such pressure is produced by the partially dehydrated NP gel, which is trying to absorb water to regain equilibrium and increase its volume. Swelling pressure is also a key point in the function of the intra-vertebral spinal disc. When the axial pressure load increases, part of the liquid is expelled from the nucleus, which increases the swelling pressure (the concentration of the polymer increases). The process will stop only when equilibrium is reached and the axial pressure equals the swelling pressure. In this way, the NP is able to balance out and apportion the pressure in the spine, and it does so especially by transferring axial load into radial load, which is then captured by the AF. Furthermore, the changes in swelling are the driving force of the transportation of metabolites and nutrients, without which the NP tissues could not survive in the long term.
From the above-stated facts it is obvious, that the construction of a fully functional prosthesis is extremely difficult. Most prosthetic devices suggested to date are strictly mechanical, and they mimic only some functions of the disc. An example of such are constructions described in U.S. patents by Froning (U.S. Pat. No. 3,875,595), Kuntz (U.S. Pat. No. 4,349,921), Shepperd (U.S. Pat. No. 4,863,476), Olerud (U.S. Pat. No. 5,053,034), Bryan et al. (U.S. Pat. Nos. 5,674,269 and 5,865,846), Yuan et al. (U.S. Pat. No. 5,676,701) and Serhan, et al. (U.S. Pat. No. 5,834,094).
A more perfect prosthetic with a truer simulation of the disk function was suggested in a U.S. Pat. No. 4,911,718 “Functional and Biocompatible Intervertebral Spacer” (Lee et al, 1990), describing a composite construction of the prosthetic of the disc using a biocompatible elastomer, reinforced by fibers which mimic the function of collagen fibers in a natural spinal disc. The main disadvantage of this solution, which is common to all full spinal disc replacements, remains a complicated surgical procedure, which translates into a high cost, and a high risk to the patient.
In many cases, only the NP (or even a portion of the NP) can be replaced to restore function. The replacement of a missing NP will keep the AF in a necessary state of tension, which will also enable its proper mechanical function. It is also necessary to replace the function of the original NP in regards to nutrient and metabolic waste transport, since without this feature the remaining living tissues of the spinal disc cannot survive. For that reason, the NP substitute must be made of a hydrogel with a sufficient swelling pressure and a capability of hydraulic transport of fluids.
A hydrogel substitute for the NP was first suggested by Bao et al in the U.S. Pat. No. 5,047,055. Bao describes a hydrogel prosthesis of the nucleus, whose shape and size corresponds to the removed disc nucleus when the prosthesis is fully swollen. According to the requirements stated in the patent, the hydrogel used in a fully swollen state must have a water content of at least 30% by weight, and a pressure strength of at least 4 MN.m−2. This high strength must be achieved even at full swelling in water and during a high water content (Bao suggests 70% to 90% by weight as optimum). This high strength is apparently requested in order to prevent isotropic extrusion of the material implanted into the damaged and weakened AF. The selection of hydrogels fulfilling this requirement is narrow, however.
Furthermore, Bao teaches to implant his prosthetic in a partially dehydrated state when the dimensions are smaller and the device can be inserted through a smaller opening. After implantation, the prosthetic will grow to its full size by absorbing bodily fluids. It is necessary to note, however, that the dehydration prior to implantation and rehydration after implantation are isotropic, i.e. all dimensions change at the same rate. This can be seen as a significant disadvantage of the Bao concept. During implantation the implant will try to expand equally in all directions, but it will expand most in the direction of the least resistance. Therefore it will expand the least in the axial direction, where expansion is most needed (so that the separation of the vertebrae is the highest), and it will expand the most in the radial direction, where the expansion is least desirable; especially in places where the AF is weakened or even missing. For this reason Bao was forced to suggest an implant which is in its fully swollen state exactly the size of the cavity created by the removal of the NP or its part. This presents a significant obstacle, however, and also means a lowered function of the implant, which will then will have a zero swelling pressure at a fully swollen state.
Another disadvantage, stemming from the isotropic characteristics of the materials, is its radial bulging under axial pressure. This bulging will be the greatest in the direction of the least resistance, i.e. in the direction where the AF is weakened or damaged. The implant can also have the tendency to slowly change shape and herniate when subject to permanent and long-term pressure. The high strength and modulus requirements which Bao states are apparently governed by the effort to prevent such undesirable deformation.
Some of the stated deficiencies were later remedied in the following U.S. Pat. No. 5,192,326 (Bao et al). In this case the prosthetic is formed by hydrogel spheres placed within an elastic, semi-permeable sack or wrap. The porous wrap, in its fully unfurled state, has the shape and size of the cavity created by the removal of the NP or its part. The size of hydrogel spheres is at least three times the size of the size of the pore, therefore they cannot escape out of the wrap. The hydrogel can contain up to 99% fluid by weight in its fully swollen state, and its strength doesn't need to be as high as in the previous case since the strength of the implant is given by the strength of the wrap and not of the hydrogel. The volume of the fully swollen hydrogel cannot be larger than the size of the cavity created by the removal of the NP, since the full swelling is prevented by the resistance of the wrap against further expansion.
A similar implant is described by Ray et al. in U.S. Pat. No. 4,772,287. Ray describes an implant into NP, consisting of two cylindrical bladders filled with a fluid. The bladders are enclosed in a fibrous wrap.
In a U.S. Pat. No. 4,904,260, Ray describes an improvement upon the previous invention, which relies on the bladders being semi-permeable, and the liquid in it contains a substance with a therapeutic effect. This substance is capable of slowly diffusing from the prosthetic into the surrounding tissue.
A further improvement is described by the same author in a U.S. Pat. No. 5,674,295. Here, the bladders filled with liquid are replaced by cylindrical hydrogel bodies. The solid fibrous wrap allows for higher swelling in the axial direction and prevents excessive swelling in the radial direction, thus protecting the AF from bulging due to the swelling pressure of the implant.
This invention is further modified in a U.S. Pat. No. 5,824,093 where the hydrogel bodies have an oval rather than a circular cross-section, and their wrap is constructed so that their general shape is maintained even during full swelling and loading. The implants described by Ray are not an actual replacement of the NP, because they have significantly different shape and characteristics. More specifically, this is a device for a partial fusion and partial immobilization of the spinal disc, rather than the restoration of its function.
The above overview indicates that a fully optimal NP prosthetic was not yet described.